The present invention relates generally to a load current harmonics neutralizing apparatus for interfacing a critical load with an AC power source, and more particularly to an active harmonic filtering system and a standby power supply (SPS) system for powering computer, communication, medical and other equipment, during both normal and emergency conditions, such as utility power line outages.
A revolution in the fields of microelectronics technology and the information sciences has led to the widespread use of critical electrical loads, such as in the communication and medical fields. Another recent revolution in power electronic technologies has resulted in the widespread use of nonlinear loads, including personal computers, adjustable speed drives for air conditioners and heat pumps, uninterruptible power supplies, high frequency fluorescent lights, microwave ovens and various other office and consumer electronics equipment. This trend is expected to continue in the future.
The problem of static power converters creating harmonic currents on power systems was first addressed by the IEEE in 1981 with the introduction of the Standard IEEE No. 519-1981, entitled "IEEE Guide for Harmonic Control and Reactive Compensation of Static Power Converters." Many of the users of the new non-linear electric loads did not have electrical engineers on staff to even recognize the harmonics problems, so the IEEE standard established a performance baseline.
In an article by C. K. Duffey and R. P. Stratford entitled, "Update of Harmonic Standard IEEE-519, IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems," IEEE Paper No. PCIC-88-7 (IEEE, 1988), a large industrial plant was studied, and the guidelines from Standard IEEE-519 were applied. Half of the plant's load was attributed to a twelve-pulse static power converter. The equivalent harmonic current distortion of the odd harmonics above the seventh (11, 13, 23, 25, 35, etc.) were higher than the baseline of IEEE-519. If there are other users on the line, the article noted that the utility system may be distorted beyond that allowed by the standard.
While harmonic filters could be placed on the line to reduce the current and voltage distortion to within acceptable limits, such harmonic filters are costly, both in terms of initial cost and operating cost. The Duffey/Stratford article concluded that the most effective way to correct harmonic distortion is where the customer couples with the electric utility. Without harmonic correction, the increased use of non-linear electric loads threatens to deteriorate the high quality power for which North America is well known.
Many nonlinear loads operate on single-phase alternating current (AC) power received from an electric utility. During a utility power outage, a battery backup is often required to avoid the loss of critical information, functions or services. A variety of UPS systems have been designed to meet these emergency power needs.
Conventional UPS systems provide an interface between the utility power system input and a critical load (personal computers, communication and medical equipment, and other loads normally receiving AC power and requiring battery backup power will herein be referred to as "critical loads"). Basically, the AC power from the utility is first rectified, and a small portion of the rectified power is used to charge the backup battery. The remaining portion of the rectified power is supplied to the critical load through an inverter, which converts the DC voltage to nearly sinusoidal line or fundamental frequency (e.g., 50 Hz in Europe or 60 Hz in the United States) voltage, to the critical load. During a utility power outage emergency condition, power is supplied by the battery through the inverter to the load.
Conventional UPS systems typically have a rectifier/charger which receives AC power from an AC power utility through a static interrupter and rectifies it into DC power. A portion of the rectified DC power charges a battery bank during normal operating conditions. A line-frequency isolation transformer is often included between the interrupter and the critical load. During normal operating conditions, the interrupter is closed to allow the AC power to flow therethrough to a critical load.
During a utility power outage, the static interrupter opens to isolate the load from the utility. The battery bank powers the load through a pulse width modulated (PWM) DC-to-AC inverter. Line-frequency isolation transformers may be placed between the inverter and the load. Thus, power is maintained to the load during both normal and emergency operating conditions. Alternatively, during normal operation, power is supplied to the load through the rectifier/charger, the inverter and an isolation transformer. In this alternate embodiment, the AC power supplied through the interrupter and isolation transformer (if used) is merely an additional backup feature to further enhance the reliability of the UPS system.
The known conventional UPS systems suffer several significant drawbacks. For example, the line-frequency isolation transformers are bulky, expensive and introduce substantial power losses into the UPS system.
The conventional UPS systems disadvantageously inject undesirable current harmonics into the utility system. These injected current harmonics produce line voltage distortion which interferes with other loads connected to the same line voltage supply. The rectifier/charger and the inverter are each sources of these undesirable current harmonics. However, the inverter harmonic currents are decoupled from the utility by the rectifier/charger and by using an isolation transformer. An active filtering system to actively neutralize harmonic currents injected by an AC-to-DC converter into an AC power transmission system is disclosed in U.S. Pat. Nos. 4,053,820 and 4,224,660 (one inventor of the present invention being a co-inventor and the sole inventor, respectively, of these patents).
Another significant source of undesirable current harmonics is the critical load itself, which is often some form of a DC power supply. Conventionally, the input section of a rectifier/charger is a diode-bridge rectifier. The electrical characteristics at the input of this diode-bridge rectifier are identical to the input characteristics of the critical load. Therefore, the current drawn from the utility by a conventional UPS system comprises large amplitudes of harmonic currents. These harmonic currents can significantly interfere with other loads on the electric power line and with nearby telephone equipment. Additionally, these harmonics cause unnecessary heating of these other loads, and often contribute to a malfunction of ripple control systems within these other loads.
A variation of the above UPS system arrangement is known as a standby power supply (SPS) system. Under normal conditions, the SPS system delivers AC power directly from the utility to the critical load. During an emergency power outage, the SPS system supplies battery power through an inverter to the critical load. As mentioned above, the highly nonlinear input characteristic of the critical load draws a load current that is rich in harmonics of the fundamental line frequency. Since the critical load is supplied directly from the utility via the SPS system under normal conditions, the current drawn from the utility by the SPS system has large amounts of harmonic components.
For example, one SPS system proposed by Kawabata is coupled in parallel with the electric utility and the non-linear load. Kawabata requires that a very large inductive reactance be coupled in series between the utility and Kawabata's SPS system. With Kawabata, if the utility line voltage is distorted, a distorted current is drawn from the utility to power the load during normal operating conditions. Kawabata's system experiences this difficulty in both single phase and three phase embodiments.
Thus, both the conventional UPS and SPS systems disadvantageously draw undesirable harmonic current components from the utility system.
High technology electrical loads typically have a highly nonlinear input characteristic, that is, the input current drawn by these loads is rich in harmonics of the fundamental line frequency. For example, the lower-order harmonics, such as the third and fifth harmonic current components, can have very large magnitudes. With single phase nonlinear loads, the amplitude of the third harmonic component may come close to the amplitude of the fundamental frequency current component.
In a three phase, four wire power supply system, such as the service used to supply large residential, commercial and office buildings, the phase to neutral voltages are distributed to the various building floors in an attempt to balance the load on each of the phases. The third harmonic current components from each phase are combined and carried on the fourth neutral wire. These large harmonic currents on the neutral conductor result because the zero-sequence harmonic currents on the phase conductors add up on the neutral wire, rather than cancelling each other out. In several cases, the neutral wire itself has heated to the extent of becoming a fire hazard.
One possible expensive solution to neutral overheating is to replace the neutral wire with a larger conductor, but far more damaging is the effect of the neutral wire harmonic currents on the customer's distribution transformer and the utility's substation power transformer. The compounding of the neutral conductor harmonics currents from the various utility customers can cause the substation transformer to become overloaded and overheated, which may ultimately lead to transformer failure and a power outage for the customers.
To avoid a transformer burnout, the substation transformer may be replaced by a transformer having a higher current rating, but this is a very expensive solution. Another approach proposed in the past uses three single phase active filters at the four wire service entrance to each building. Each active filter is connected between a phase line and the neutral line to supply the harmonic currents drawn by the load so the harmonics are not drawn from the utility source through the substation transformer. Such single phase active filters are costly, both in terms of their initial installation, and their electrical losses during operation.
For the three phase four wire systems, in the past, the harmonics have been filtered using single phase passive filters, active filters and hybrid approaches. The passive approaches are simple, but often bulky and expensive. Moreover, the passive filters have components sensitive to temperature and aging which lead to ineffective filtering as the critical frequencies and Q-values drift. Using passive approaches, the danger exists of exciting a resonance condition with the AC system impedance, which can worsen, rather than alleviate the harmonic problem.
The active filter approaches proposed in the past have used three single phase active filters, which has proved very effective. However, these earlier systems are costly, requiring twelve controlled switches to operate. Moreover, these earlier three phase three wire active filter arrangements cannot be used in a three phase four wire system to eliminate harmonics on the neutral wire.
Thus a need exists for an improved load current harmonics neutralizing apparatus for interfacing a critical load with an AC power source, and more particularly to an active harmonic filtering system and a standby power supply (SPS) system, which are directed toward overcoming, and which are not susceptible to, the above limitations and disadvantages.